1. Field of the Invention
The present invention relates to water repellent glass in which a water repellent film is formed on a surface of its glass substrate and to a process for producing the same. Water repellent glass according to the present invention can be applied to automobile windshield glass, mirror or the like.
2. Description of the Related Art
Water repellent glass has been investigated extensively in order to repel water droplets deposited on automobile windshield glass, side mirror or the like and in order to make vision area favorable during rainy weather. For example, it is possible to think of making glass itself water repellent. At present, however, such an idea has not been put to practical application yet, because there are many problems to be solved technically. Accordingly, many have come to think of forming a water repellent transparent film on a surface of a glass substrate. There have been a variety of proposals on this idea.
For instance, there has been proposed water repellent glass which has a resin film made of silicone resin, such as polydimethylsiloxane or the like, on a surface of its glass substrate. Since the silicone resin has a very favorable water repellency, this water repellent glass has an excellent water repellency.
Moreover, in Journal of Non-Crystalline Solids 121 (1990) 344-347, there is set forth a process for forming a water repellent film which comprises fluorine-including ZrO.sub.2 on a steel plate. The water repellent film is made from a reaction product of a trace amount of trimethoxyfluoroalkylsilane and ZTO (zirconium tetraoctylate).
In the water repellent glass having the silicone resin film, however, the water repellent film is insufficient in terms of a weather resistance, and it offers a short life when it is put to practical application. It has been found that the cause of this drawback results from the following mechanism. Namely, the silicone resin film allows water to transmit through itself. Accordingly, water transmits through the water repellent film, and it reaches the glass substrate, thereby solving the alkaline components contained in the glass substrate out. In addition, it is hard to say that the silicone resin has a favorable resistance against alkali. As a result, the silicone resin is hydrolyzed by the alkaline components, thereby deteriorating the water repellency.
Moreover, although the water repellent film comprising the fluorine-including ZrO.sub.2 has a water repellency, it provides less affinity for glass and accordingly it is likely to come off when it is formed on a glass substrate. In addition, the fluorine-including ZrO.sub.2 has an extremely high refractive index as high as 1.9 approximately. Consequently, when a thin film made of the fluorine-including ZrO.sub.2 is formed on a glass substrate, the reflection lights are colored by the light interference. Therefore, the water repellent glass having the fluorine-including ZrO.sub.2 water repellent film is not appropriate for automobile application. For instance, when the fluorine-including ZrO.sub.2 water repellent film is formed on a soda-lime glass surface in a film thickness of 180 nm, there arises a reflection spectrum as illustrated in FIG. 4.